In a radio frequency (RF) transceiver, a communication signal is typically amplified and transmitted by a transmit section. A transmit section may comprise one or more circuits that amplify and transmit the communication signal. The amplifier circuit or circuits may comprise one or more amplifier stages that may include one or more driver stages and one or more power amplifier stages. Each of the amplifier stages typically comprises one or more transistors configured in various ways to amplify the communication signal.
One of the challenges in designing a power amplifier in an RF transmitter is managing intermodulation distortion (IM or IMD). Intermodulation can be described as the amplitude modulation of a signal containing two or more frequencies caused by nonlinearities in a system. The intermodulation between each frequency component will form additional signals at frequencies that are not just at harmonic frequencies (integer multiples) of either signal, but also at the sum and difference frequencies of the original frequencies and at multiples of those sum and difference frequencies. One intermodulation component that may be particularly destructive is the third order intermodulation product, also referred to as IM3 that gives rise to two-tone third order intermodulation distortion, IMD3. Two-tone IMD3 is the measure of the third-order distortion products produced by a nonlinear device when two tones closely spaced in frequency are provided into its input. This distortion product is usually located sufficiently close to the carrier frequency so as to be difficult to filter out and can cause interference in multichannel communications equipment.
It is desirable to implement improved RF amplifiers and/or to reduce or eliminate IM3 and IMD3 in such amplifiers.